Role of upper triplet states on the photophysics of nitrated polyaromatic compounds: S(1) lifetimes of singly nitrated pyrenes.

The photophysics of most nitrated polycyclic aromatic compounds is dominated by an ultrafast intersystem crossing channel, which makes their first singlet excited states decay with rates on the order of 10(12) to 10(13) s(-1). Some questions, however, remain about the nature of the receiver triplet states, which have been in principle assigned to specific triplets of a different electronic configuration from T(1). In particular, it could be suggested that even a small degree of n-π* character of the T(1) state may be enough to allow the S(1) state to couple to upper vibronic states of the lowest energy triplet, without the requirement for specific upper triplet states. In this report, we show that there are, in fact, nitroaromatic compounds that do not show the ultrafast intersystem crossing channel but instead have S(1) states that are two to three orders of magnitude longer lived. Our studies focused on the time resolution of the emission from singly nitrated pyrenes, which show a strong photophysical dependence on the position of the NO(2) group: Whereas S(1) in 1-nitropyrene is short-lived (up to 3 ps), in 4-nitropyrene and 2-nitropyrene this state has 0.41 and 1.2 ns lifetimes, respectively, in acetonitrile solution. Computational work at the TD-DFT level of theory indicates that such remarkable increase in the first excited singlet lifetime can indeed be explained by a loss of the energy coincidence between the S(1) state with specific upper triplet states formed from transitions that involve the nonbonding orbitals at the oxygen atoms. These results are in strong support of the previous descriptions about the requirement for intermediacy of specific triplet states in the ultrafast decay of the fluorescent state present in most nitroaromatics. The implications for the photochemistry of this group of toxic atmospheric pollutants, including the channel that redounds in the dissociation of the NO· fragment, are discussed in view of the present results.

Primary photochemistry of nitrated aromatic compounds: excited-state dynamics and NO· dissociation from 9-nitroanthracene.

We report results of femtosecond-resolved ex-periments which elucidate the time scale for the primary photoinduced events in the model nitroaromatic compound 9-nitroanthracene. Through time-resolved fluorescence measurements, we observed the ultrafast decay of the initially excited singlet state, and through transient absorption experiments, we observed the spectral evolution associated with the formation of the relaxed phosphorescent T(1) state. Additionally, we have detected for the first time the accumulation of the anthryloxy radical which results from the nitro-group rearrangement and NO(•) dissociation from photoexcited 9-nitroanthracene, a photochemical channel which occurs in parallel with the formation of the phosphorescent state. The spectral evolution in this molecule is highly complex since both channels take place in similar time ranges of up to a few picoseconds. Despite this complexity, our experiments provide the general time scales in which the primary products are formed. In addition, we include calculations at the time-dependent density functional level of theory which distinguish the molecular orbitals responsible for the n-π* character of the "receiver" vibronic triplet states that couple with the first singlet state and promote the ultrafast transfer of population between the two manifolds. Comparisons with the isoelectronic compounds anthracene-9-carboxylic acid and its conjugated base, which are highly fluorescent, show that in these two compounds the near-isoenergeticity of the S(1) with an appropriate "receiver" triplet state is disrupted, providing support to the idea that a specific energy coincidence is important for the ultrafast population of the triplet manifold, prevalent in polycyclic nitrated aromatic compounds.

Excited-state dynamics of nitrated push-pull molecules: the importance of the relative energy of the singlet and triplet manifolds.

We present a study of the dynamics following photoexcitation in the first electronic band of NO(2)-para-substituted nitronaphthalenes. Our main goal was to determine the interplay between the nitro group, electron-donating substituents, and the solvent in defining the relative excited-state energies and their photoinduced pathways. We studied 4-nitro-1-naphthylamine and 1-methoxy-4-nitronaphthalene in solution samples through femtosecond fluorescence up-conversion and transient absorption techniques. In all solvents, both compounds have ultrafast fluorescence decays, showing that, similarly to the parent compound 1-nitronaphthalene, these molecules have highly efficient S(1) decay channels. The evolution of the transient absorption signals in the visible region reveals that for the methoxy-substituted compound, independently of solvent polarity, the photophysical pathways are the same as in 1-nitronaphthalene, namely, ultrafast intersystem crossing to an upper triplet state (receiver T(n) state) followed by relaxation into the lowest energy phosphorescent triplet T(1). In contrast, for the amino-substituted nitronaphthalene, the excited-state evolution shows a strong solvent dependence: In nonpolar solvents, the same type of intersystem crossing through an upper receiver triplet state dictates the photochemistry. However, in methanol, where the first singlet excited state shows an important solvent-induced stabilization, we observed typical signals of the repopulation of the electronic ground state in the time scale of less than 1 ps followed by vibrational cooling within S(0). Excited-state calculations at the time-dependent density functional level with the PBE0 functional give an approximate characterization of the states involved and appear to correlate well with the experimental results as they show that the S(1) state of the amino compound is stabilized with respect to upper triplet states only in the polar solvent. These findings sustain and illustrate the recent view that the intersystem crossing channel so prevalent in nitroaromatic compounds is related to an energy coincidence between the pi-pi* first singlet excited state and upper triplet states with n-pi* character. Our results indicate through direct observations that if the S(1) state is sufficiently stabilized, other rapid decay channels like internal conversion to the ground state will minimize the transfer of population to the triplet manifold.

Relaxation in the triplet manifold of 1-nitronaphthalene observed by transient absorption spectroscopy.

Previous phosphorescence and triplet quantum yield determinations indicate that the primary photophysical channel for 1-nitronaphthalene is the formation of its lowest energy triplet state. Also, previous direct measurements of the decay of the fluorescence from this compound indicated that the crossing between the singlet and triplet manifolds is ultrafast (sub-100 fs). In this contribution we present a sub-picosecond transient absorption study of the relaxation of photoexcited 1-nitronaphthalene in methanol and other solvents. Our measurements reveal the time scale in which the fully relaxed T(1) state is formed. We have observed that the spectral evolution associated with this process takes place in time scales from one to a few tens of picoseconds. Specifically, the appearance of the absorption spectrum of T(1) in the visible region is accompanied by the decay of transient signals at wavelengths below 400 nm. Since the fluorescence lifetime of this compound is sub-100 fs, we assigned the picoseconds decaying signals below 400 nm to an intermediate triplet state which acts as a receiver state in the intersystem crossing step and from which the T(1) population accumulates. From the details of the spectral evolution and the effects of different solvents, we also conclude that T(1) formation and vibrational cooling within this state occur in similar time scales of between 1 and 16 ps. Mainly, our results provide direct evidence in support of the participation of an upper triplet state in the mechanism for intersystem crossing in this molecule. This is considered to be common in the photophysics of several nitrated polycyclic aromatic compounds and the most determinant feature of their primary photochemistry.